1. Field of the Invention
This invention relates to a composition for use in forming shaped friction elements such as clutch facings, brake pads or linings, and the like. The invention permits such friction elements to possess properties comparable with known friction elements yet, consonant with the increased demand to protect workers against exposure to airborne substances that are deleterious to their health, eliminates asbestos as the conventional components of such friction elements.
2. Description of the Prior Art
The mineral asbestos has been long associated with the manufacture of articles whose use requires that they withstand heat. Even before the days of Marco Polo, who reported to fellow Venetians upon his travels to Siberia where he saw the fibrous substance woven into cloth which remained incombustible when thrown into fire, the heat-resistive property of asbestos was known. It is not surprising, therefore, that asbestos has been the major component of friction elements such as those used in the brake and clutch assemblies of automotive vehicles where severe operating temperatures and pressures must be withstood. U.S. Pat. Nos. 1,608,165; 2,025,039; 2,218,535; 2,431,883; 2,702,770; 3,068,131; and 3,437,546 are representative of the period, spanning almost 50 years, that asbestos dominated as the principal active friction ingredient in friction materials.
When the asbestos used in friction elements is in the form of a yarn, the yarn is commonly provided with a core of fine metal wire to provide the tensile strength needed in handling the yarn during fabrication of a friction element. Workers in the art found it necessary, however, to reinforce the asbestos fibers with a stronger fibrous substance such as cotton in order to spin the asbestos into a yarn. These workers thus commonly preferred a yarn composed of about 75 to 85 percent asbestos and 15 to 25 percent cotton; see e.g. U.S. Pat. Nos. 2,052,808; 2,130,520; 2,855,081; 3,068,131; 3,365,041; and 3,429,766. The teachings of these patents make clear that the introduction of cotton or some other fiber into the asbestos was to improve its spinability or the tensile strength of the yarn, and thus the addition was to be tolerated rather than encouraged. In fact, the need for including a limited amount of other fibers such as cotton in asbestos yarn intended for use in friction elements has become so well accepted that more recent patent disclosures refer only to "asbestos yarn" without specifically mentioning such other fibers; see e.g. U.S. Pat. Nos. 3,437,546 and 3,600,258. Other teachings have suggested the use of cellulosic fibers in combination with asbestos to minimize the undesirable characteristic of "fading" under the frictional heat of operation; see U.S. Pat. No. 2,702,770.
Since the passage of the Occupational Safety and Health Act of 1970, the Occupational Safety and Health Administration (OSHA) has set standards for occupational exposure to asbestos and it appears that these standards will become increasingly rigid to the point where zero exposure may be required. The present OSHA standards, among other things, limit the number of asbestos fibers per unit volume of air to which a worker may be exposed over an established period of time. The imposition of such standards was brought about by evidence that exposure to asbestos may be carcinogenic to man.
Asbestos dust probably is present at highest concentrations, with the exception of asbestos mines, in asbestos textile plants where the asbestos fibers are prepared, carded, spun, woven, etc. However, asbestos dust is also present in plants at which friction materials are fabricated. Depending upon particular plant conditions, operations such as mixing, forming, pressing and baking asbestos containing materials, grinding, sanding, cutting and drilling asbestos-containing articles, and bonding, riveting, inspecting and packing finished asbestos-containing friction elements may all contribute to the presence of airborne asbestos. It is thus a worthy (and perhaps in the future a mandatory) objective to eliminate asbestos as a component of friction materials.
The use of glass fibers in friction products has been suggested. Some of the earlier of these suggestions involved the use of glass fibers to reinforce asbestos-containing friction elements, first as backing materials for conventionally produced friction elements (e.g. U.S. Pat. Nos. 3,068,131 and 3,365,041), and then as part of the friction facing itself (e.g. U.S. Pat. Nos. 3,429,766; 3,526,306; 3,520,390; and 3,600,258). The stated purpose for the use of glass fibers was to increase the burst strength of the friction elements (Burst strength is an index of the centrifugal forces which can be withstood by a friction element without disintegrating. The test of burst strength is usually carried out at elevated temperatures).
More recently, it has been suggested that glass fibers alone or together with metal wire or chips may be used to form friction elements containing no asbestos (e.g. U.S. Pat. Nos. 3,743,069 and 3,967,037). In these cases, it is stated that the glass fibers not only improve burst strength and wear-resistance, particularly at elevated temperatures, but also serve as the active friction ingredient. Generally, such friction elements are fabricated by known techniques using a conventional, heat-curable organic binder to bond the glass fibers together in a mass.
It has been found however, that glass, as the active friction ingredient of a friction element, is too "aggressive" in most commercial applications in comparison with conventional asbestos-containing friction facings. This aggressiveness is often manifested during simulated or actual operating conditions as noise, vibration, and/or erratic friction effects when the friction element is engaged with a mating surface, none of which manifestations can be tolerated by the highly-competitive automotive industry.
The present invention overcomes the shortcomings of known compositions for friction elements in two ways: (1) the invention provides a friction material composition which does not require asbestos as a component, yet produces operating characteristics comparable with those of asbestos-containing friction elements; and (2) the invention permits the use of glass as an active friction ingredient of friction elements without the attendant operational shortcomings described above.